Generally, an organic light emitting display (hereinafter, referred to as OLED) is an emissive display device emitting light by electrically exciting a fluorescent organic compound. OLEDs may be classified as a passive matrix type and an active matrix type depending on how the pixels arranged in an N×M matrix are driven. An active matrix OLED has advantages in that it can be used for a large screen and high-resolution display since it has low power consumption compared to a passive matrix OLED. In addition, OLEDs may be classified as top emission type, bottom emission type, and double-sided emission type depending on the direction light is emitted from the organic compound. A top emission OLED has an advantage of a high aperture ratio for a device emitting light in the reverse direction from the substrate including the unit pixels, unlike a bottom emission OLED.
The organic compound for emitting the OLED is formed at an emission region of a pixel electrode which is an anode. The organic compound is formed by a laser-induced thermal imaging (LITI) method, a low molecular deposition method, or the like.
FIG. 1 is a cross-sectional view illustrating a conventional OLED, which will be briefly described with reference to a method of fabricating the same.
First, a buffer layer 110 is formed to a predetermined thickness on a substrate 100, and then a thin film transistor (TFT) including a polysilicon pattern 120, a gate electrode 132, and source and drain electrodes 150 and 152 is formed. At this time, source and drain regions 122, in which impurities are implanted, are formed at both sides of the polysilicon pattern 120, and a gate insulating layer 130 is formed on an entire surface including the polysilicon pattern 120. An interlayer dielectric 140 is formed over the gate electrode 132 and the gate insulating layer 130.
Next, a passivation layer 160 is formed to a predetermined thickness on the entire surface, and then the passivation layer 160 is etched by an etching process to form a first via contact hole 162 for exposing at least one of the source and drain electrodes 150 and 152, for example, the drain electrode 152. The passivation layer 160 is made of an inorganic insulating layer such as a silicon nitride layer, a silicon oxide layer, or a stacked structure thereof.
Then, a planarization layer 170 is formed on the entire surface of the structure. The planarization layer 170 may be formed of a material selected from the group consisting of polyimides, benzocyclobutene-based resins, spin on glass (SOG), and acrylates, which is formed to planarize the emission region.
Continuously, the planarization layer 170 is etched by photolithography and etching processes to form a second via contact hole 172 for exposing the first via contact hole 162.
Next, a thin layer (not shown) for forming a pixel electrode 180 is formed. The thin layer is formed to a thickness of about 10˜1500 Å using a transparent metal material such as ITO (indium tin oxide).
Next, the thin layer is etched by photolithography and etching processes to form a pixel electrode 180. In this process, when a reflection layer is formed under the pixel electrode 180, a top emission OLED is formed. On the other hand, when the reflection layer is formed in forming the opposite electrode, a bottom emission OLED is formed. The thickness of the thin layer for the pixel electrode may also be varied depending upon the kind of OLED.
Then, a pixel defining layer pattern 190 for defining the emission region is formed on the entire surface of the structure. The pixel defining layer pattern 190 may be formed of a material selected from the group consisting of polyimides, benzocyclobutene-based resins, phenol-based resins, and acrylates.
Subsequently, an organic layer 192 including at least one emission layer is formed on a pixel region defined by the pixel defining layer pattern 190 using a low molecular deposition method or an LITI method. Then, an opposite electrode 194 is formed to complete the OLED. At this time, for a top emission OLED, the opposite electrode is formed of a transparent electrode or a transparent metal electrode, and in the case of a bottom emission OLED, the opposite electrode is formed of a metal electrode having a reflection layer or a reflection electrode.
In the method of fabricating an OLED in accordance with the conventional art as described above, the thin layer for the pixel electrode uses a transparent metal thin layer such as ITO or IZO, and it is advantageous to form the pixel defining layer to a thickness of not more than 3000 Å. Here, while it is impossible to wet-etch polycrystalline ITO among ITO materials used for the thin layer for the pixel electrode, amorphous ITO can be wet etched. However, an etching surface of the amorphous ITO has a vertical or undercut portion to form a step at an edge of the pixel electrode as shown in {circle around (x)} portion of FIG. 1. As a result, there is a high probability that a lower portion of the etching surface of the pixel electrode is not buried in the following process of forming the pixel defining layer, and the pixel defining layer may be damaged by pressure applied when the organic layer including the emission layer is formed. The damage to the pixel defining layer may cause a short-circuit between the pixel electrode and the opposite electrode, increasing the chance of failure of the device, and thereby lowering the yield. In addition, the pixel defining layer is formed to have a small thickness at an upper portion of the etching surface of the pixel electrode to cause an electric field to be concentrated between an upper portion of an edge of the pixel electrode having a vertical etching surface and the opposite electrode, thereby deteriorating the organic layer and reducing the lifetime of the OLED.